The present invention refers to polystyrene (PS) and poly-lactic acid (PLA) blends having as compatibilizing agent a PS-PLA block copolymer. Due to the thermodynamic incompatibility between PS and PLA, their blends result in heterogeneous materials with inadequate mechanical, thermal and processability properties for the intended applications; this can be reversed by adding a compatibilizing agent, as described in this invention patent application